The aim of the proposed research is to study the influence of the immunological system on development of malignancy. The chicken has been chosen as the experimental animal because of the unique delineation of immunological responsiveness in cellular, thymus-dependent immunity, on the one hand, and humoral immunity, dependent on the bursa of Fabricius, a hindgut lymphoid organ present in the birds, on the other. The chicken model offers the opportunity to study the influence of cellular and humoral immunity on tumorigenesis separately by manipulations of the thymus and of the bursa of Fabricius. Thymectomy experiments are designed to evaluate the possible role played by cellular, thymus- dependent immunity in surveillance and other immunological tumor- controlling mechanisms, and bursectomy experiments to study the role of antibody-mediated tumor control mechanisms, such as enhancement and cytotoxicity. Experiments combining thymectomy and bursectomy will also give information on tumor development in birds void of essentially all humoral and cellular immunity. The malignancy caused by the reticuloendotheliosis (RE) virus will be used as the tumor system in these studies. This disease of the avian reticuloendothelial system has the histopathological signs of malignancy, and is 100% lethal within a relatively short period of time. The tumors are first found in the spleen and in the liver, and later in all organs where the reticuloendothelial system is represented. The thymus and the bursa of Fabricius are among the last to be affected. The relatively simple experimental model may provide more clear-cut information pertaining to the role of the immunological system in tumorigenesis than is presently available.